.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
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.. c:namespace:: V4L
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.. _func-select:
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*************
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V4L2 select()
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*************
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Name
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====
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v4l2-select - Synchronous I/O multiplexing
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Synopsis
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========
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.. code-block:: c
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#include <sys/time.h>
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#include <sys/types.h>
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#include <unistd.h>
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.. c:function:: int select( int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout )
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Arguments
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=========
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``nfds``
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The highest-numbered file descriptor in any of the three sets, plus 1.
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``readfds``
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File descriptions to be watched if a read() call won't block.
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``writefds``
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File descriptions to be watched if a write() won't block.
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``exceptfds``
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File descriptions to be watched for V4L2 events.
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``timeout``
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Maximum time to wait.
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Description
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===========
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With the :c:func:`select()` function applications can suspend
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execution until the driver has captured data or is ready to accept data
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for output.
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When streaming I/O has been negotiated this function waits until a
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buffer has been filled or displayed and can be dequeued with the
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:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. When buffers are already in
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the outgoing queue of the driver the function returns immediately.
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On success :c:func:`select()` returns the total number of bits set in
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``fd_set``. When the function timed out it returns
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a value of zero. On failure it returns -1 and the ``errno`` variable is
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set appropriately. When the application did not call
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:ref:`VIDIOC_QBUF` or
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:ref:`VIDIOC_STREAMON` yet the :c:func:`select()`
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function succeeds, setting the bit of the file descriptor in ``readfds``
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or ``writefds``, but subsequent :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`
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calls will fail. [#f1]_
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When use of the :c:func:`read()` function has been negotiated and the
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driver does not capture yet, the :c:func:`select()` function starts
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capturing. When that fails, :c:func:`select()` returns successful and
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a subsequent :c:func:`read()` call, which also attempts to start
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capturing, will return an appropriate error code. When the driver
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captures continuously (as opposed to, for example, still images) and
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data is already available the :c:func:`select()` function returns
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immediately.
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When use of the :c:func:`write()` function has been negotiated the
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:c:func:`select()` function just waits until the driver is ready for a
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non-blocking :c:func:`write()` call.
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All drivers implementing the :c:func:`read()` or :c:func:`write()`
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function or streaming I/O must also support the :c:func:`select()`
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function.
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For more details see the :c:func:`select()` manual page.
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Return Value
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============
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On success, :c:func:`select()` returns the number of descriptors
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contained in the three returned descriptor sets, which will be zero if
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the timeout expired. On error -1 is returned, and the ``errno`` variable
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is set appropriately; the sets and ``timeout`` are undefined. Possible
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error codes are:
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EBADF
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One or more of the file descriptor sets specified a file descriptor
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that is not open.
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EBUSY
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The driver does not support multiple read or write streams and the
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device is already in use.
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EFAULT
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The ``readfds``, ``writefds``, ``exceptfds`` or ``timeout`` pointer
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references an inaccessible memory area.
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EINTR
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The call was interrupted by a signal.
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EINVAL
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The ``nfds`` argument is less than zero or greater than
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``FD_SETSIZE``.
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.. [#f1]
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The Linux kernel implements :c:func:`select()` like the
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:c:func:`poll()` function, but :c:func:`select()` cannot
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return a ``POLLERR``.
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